The anchor washer generally comprises an annular portion from which there extend teeth that are inclined relative to a central axis of the annular portion. The annular portion is mounted on the end portion of the coupling device and each tooth has two side faces between which there extends a surface that faces towards the thread of the tapped end portion of the duct. The coupling device is installed by engaging the end portion of the coupling device in the tapping. The coupling device is retained by one or more teeth having free ends that bear against the flank of the thread, with the other teeth being held pushed back by the thread bearing against the teeth. Installation is fast and reliable.
Nevertheless, it has been found that the teeth of anchor washers subject the thread to wear or damage leading to particles or even shavings being detached therefrom (particularly if the wall of the duct is made of a plastics material) or else it files the thread down (if the wall of the duct is made of metal), thereby polluting the fluid that is transported and running the risk of damaging a circuit element connected downstream from the coupling device. Experiments have shown that this wear or damage results from the crest of the thread being bitten into by the side edges of the surfaces of the teeth that face towards the thread and that do not have their end surfaces bearing against the flank of the thread. This surface is a sliding surface that makes contact with the crest of the thread, and normally it ought to slide thereover. The side edges are formed by a right-angled connection between the sliding surface and the side faces. Since the anchor washer is obtained by being punched out, the side edges are either of a shape that is very slightly rounded as a result of metal being cut at the beginning of punching, or else they are in the form of a sharp cutting edge as a result of the metal tearing at the end of the punching operation (depending on whether the surface that faces towards the thread is the surface that faces the punch or that faces away from the punch). Even if the teeth are less aggressive in the first configuration, the wear they generate is nevertheless too great.